Method of manufacturing boundary acoustic wave device

ABSTRACT

A method of manufacturing a boundary acoustic wave device includes the steps of forming an electrode on a first medium layer, forming a second medium layer so as to cover the electrode on the first medium layer, and forming a sound absorbing layer on an external surface of the second medium layer. The sound absorbing layer has an acoustic velocity of transverse waves that is lower than an acoustic velocity of transverse waves of the second medium layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to boundary acoustic wave devices using SH(shear horizontal) boundary acoustic waves, and more specifically, thepresent invention relates to a boundary acoustic wave device having anelectrode in the interface between a piezoelectric material and adielectric material.

2. Description of the Related Art

Various types of surface acoustic wave devices have been used in RF andIF filters for cellular phones, VCO resonators, and VIF filters fortelevisions. Surface acoustic wave devices use surface acoustic waves,such as Rayleigh waves or first leaky waves propagating along thesurface of a medium.

Since surface acoustic waves propagate along the surface of a medium,they are sensitive to the changes of the medium state. Accordingly, inorder to protect the surface acoustic wave propagating surface of themedium, the surface acoustic wave element is enclosed in a packagehaving a recess or hole formed in the region opposing the propagatingsurface. The use of the package having the recess or the hole inevitablyincreases the cost of the surface acoustic wave device. Also, since thesize of the package is larger than the size of the surface acoustic waveelement, the size of the surface acoustic wave device must be increased.

In addition to the surface acoustic waves, acoustic waves includeboundary acoustic waves propagating along the interface between solids.

“Piezoelectric Acoustic Boundary Waves Propagating Along the InterfaceBetween SiO₂ and LiTaO₃” IEEE Trans., Sonics and Ultrasonics, VOL.SU-25, No. 6, 1978 IEEE has disclosed a boundary acoustic wave deviceusing such boundary acoustic waves. The structure of the known boundaryacoustic wave device will now be described with reference to FIG. 32.

The boundary acoustic wave device 201 has the structure in which anelectrode 204 is disposed between a first medium layer 202 and a secondmedium layer 203. In this instance, an alternating electric field isapplied to the electrode 204 to excite boundary acoustic waves topropagate while their energy is concentrated on the interface betweenthe medium layers 202 and 203 and its vicinity. This boundary acousticwave device, in which an IDT is formed on a 126°-rotated Y-plateX-propagating LiTaO₃ substrate, has a SiO₂ film with a predeterminedthickness arranged over the IDT and the LiTaO₃ substrate. In thisdocument, SV+P boundary acoustic waves called Stoneley waves arepropagated. Incidentally, this document has disclosed that when the SiO₂film has a thickness of 1.0λ (λ represents the wavelength of theboundary acoustic waves), the electromechanical coupling coefficient is2%.

Boundary acoustic waves propagate with their energy concentrated on theinterface between the solids, and the bottom surface of the LiTaO₃substrate and the upper surface of the SiO₂ film hardly have any energy.The characteristics were not therefore varied by the change of thesurface state of the substrate or the thin film. Thus, the packagehaving the recess or hole is unnecessary, and the acoustic wave devicecan be thus downsized accordingly.

“Highly Piezoelectric Boundary Waves propagating inSi/SiO₂/LiNbO₃Structure” (26th EM Symposium, May 1997, pp. 53-58 [inJapanese] has disclosed SH boundary waves propagating in a[001]-Si(110)/SiO₂/Y-cut X-propagating LiNbO₃ structure. This type of SHboundary waves feature an electromechanical coupling coefficient K²higher than that of the Stoneley waves. In the use of SH boundary wavesas well as in the use of Stoneley waves, the package having the recessor hole is not necessary. In addition, since SH boundary waves are ofSH-type fluctuation, it can be considered that the strips defining theIDT or reflectors have a higher reflection coefficient in comparisonwith the case using Stoneley waves. It is therefore expected that theuse of SH boundary waves for, for example, a resonator or a resonatorfilter can facilitate the downsizing of the device and produce sharpcharacteristics.

A boundary acoustic wave device uses boundary acoustic waves propagatingwith their energy concentrated on the interface between a first and asecond medium layer and its vicinity. In this instance, the idealthickness of the first and the second medium layer is infinite. However,their thicknesses in practice are finite.

Also, the boundary acoustic wave devices of the above-mentioneddocuments undesirably produce spurious responses in their resonancecharacteristics or filter characteristics. Hence, a boundary acousticwave resonator including such a boundary acoustic wave device is liableto produce a plurality of considerable spurious responses in thefrequency region higher than the resonant frequency. Also, a filter madeof a plurality of known boundary acoustic wave resonators, for example,a ladder-shaped filter, produces a plurality of spurious responses inthe frequency region higher than the pass band, thus degrading theout-of-band attenuation disadvantageously.

This will be further described with reference to FIGS. 33 to 36. An Auelectrode was formed to have a thickness of 0.05λ on a 15° Y-cutX-propagating LiNbO₃ substrate acting as a first medium layer, and aSiO₂ film acting as a second medium layer was deposited to a thicknessof 2λ by RF magnetron sputtering at a wafer heating temperature of 200°C. A boundary acoustic wave resonator was thus produced. The electrode204 included an IDT 204A and reflectors 204B and 204C, as shown in FIG.33. The impedance-frequency characteristics and the phase-frequencycharacteristics of the boundary acoustic wave resonator are shown inFIG. 34. As designated by the arrows A1 to A3 in FIG. 34, large spuriousresponses occurred in the higher region than the antiresonant frequency.

Also, a ladder-shaped circuit shown in FIG. 35 was produced using aplurality of boundary acoustic wave resonators prepared in the samemanner as described above, and the frequency characteristics of aladder-shaped filter thus produced were measured. FIG. 36 shows theresults. In FIG. 35, parallel arm resonators P1 and P3 each included anIDT having 50.5 pairs of electrode fingers and an aperture length of30λ. Series arm resonators S1 and S2 are structured by connecting thesame two boundary acoustic wave resonators as used for the parallel armresonators P1 and P3 in series. Another parallel arm resonator P2included an IDT having 100.5 pairs of electrode fingers and an aperturelength of 30λ. Each λ of the IDTs and reflectors of the parallel armresonators P1 to P3 was 3.0 μm, and λ of the series arm resonators wasset so that the antiresonant frequency of the parallel arm resonator P1and the resonant frequency of the series arm resonators overlap witheach other. The duty ratios of the IDT and the reflectors were each0.58. The electrode was made of Au and had a thickness of 0.05λ, and theSiO₂ film had a thickness of 2.5λ.

FIG. 36 clearly shows that a plurality of large spurious responsesindicated by arrows B1 to B3 occur in a region higher than the passband.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a boundary acoustic wave device thatprevents a plurality of spurious responses in the region higher than theresonant frequency or the pass band and exhibits superior frequencycharacteristics.

Preferred embodiments of the present invention are directed to aboundary acoustic wave device using boundary acoustic waves propagatingalong the interface between a first medium layer and a second mediumlayer. The boundary acoustic wave device includes the first mediumlayer, the second medium layer stacked on the first medium layer, anelectrode disposed in the interface between the first medium layer andthe second medium layer, and a sound absorbing layer for attenuatingmodes producing spurious responses, disposed on one or both of thesurfaces of the first and second medium layers opposite the interface.

The acoustic velocity of the transverse waves in the sound absorbinglayer may be lower than the acoustic velocity of the transverse waves inthe first medium layer and/or second medium layer that has the soundabsorbing layer.

The acoustic velocity of the longitudinal waves in the sound absorbinglayer may be lower than the acoustic velocity of the longitudinal wavesin the first medium layer and/or second medium layer that has the soundabsorbing layer.

Preferably, the acoustic velocity of the transverse waves in the soundabsorbing layer is in the range of about 0.13 to about 1.23 times theacoustic velocity of the transverse waves in the first medium layerand/or second medium layer that has the sound absorbing layer.

More preferably, the acoustic impedance of the sound absorbing layer isin the range of about 0.20 to about 5.30 times the acoustic impedance ofthe first medium layer and/or second medium layer that has the soundabsorbing layer.

The sound absorbing layer may be made of the same type of material asthe first medium layer and/or the second medium layer.

The boundary acoustic wave device may further include a low attenuationconstant layer outside the sound absorbing layer. The attenuationconstant layer has a lower attenuation constant for acoustic waves thanthe sound absorbing layer.

The sound absorbing layer may be made of at least one material selectedfrom the group consisting of resin, glass, ceramic, and metal.

The sound absorbing layer may be made of a resin containing a filler.

The sound absorbing layer may be disposed on the surface of the firstmedium layer and/or the second medium layer so as to oppose a boundaryacoustic wave propagation path in the interface.

The boundary acoustic wave device may further include an electricallyconductive layer on at least one surface of the sound absorbing layer.

The boundary acoustic wave device may further include a through-holeelectrode passing through the first medium layer and/or the secondmedium layer. The through-hole electrode is electrically connected tothe electrode disposed at the interface. An external electrode is alsodisposed on an external surface of the boundary acoustic wave device andis connected to the through-hole electrode.

Preferably, the through-hole electrode is filled with an elasticmaterial.

The through-hole electrode may be provided separately in the firstmedium layer and the second medium layer, and the through-hole electrodeof the first medium layer and the through-hole electrode of the secondmedium layer are formed in a discontinuous manner.

The boundary acoustic wave device may further include a wiring electrodeprovided on an external surface of the boundary acoustic wave device andelectrically connected to the electrode disposed at the interface.

The boundary acoustic wave device may further include a connectionelectrode connected to the electrode disposed at the interface, and theboundary acoustic wave device has steps on a side surface intersectingthe interface. The connection electrode is extended to the steps, andthe wiring electrode is extended to the steps and connected to theconnection electrode at the steps.

The boundary acoustic wave device may further include a third materiallayer in at least one of regions between the first medium layer and thesecond medium layer, on the outer surface of the first medium layer, andon the outer surface of the second medium layer. The third materiallayer has a lower linear expansion coefficient in the direction parallelto the interface than the first and the second medium layer. The “outersurface” of the medium layer used herein refers to the surface oppositethe interface.

Alternatively, the boundary acoustic wave device may include a thirdmaterial layer in at least one of the regions between the first mediumlayer and the second medium layer, on the outer surface of the firstmedium layer, and on the outer surface of the second medium layer, andthe third material layer has a linear expansion coefficient in thedirection parallel to the interface, with the opposite sign to that ofthe first and the second medium layer.

The boundary acoustic wave device may further include a fourth materiallayer in at least one of regions between the first medium layer and thesecond medium layer, on the outer surface of the first medium layer, andon the outer surface of the second medium layer. The fourth materiallayer has a higher thermal conductivity than the first and the secondmedium layers.

The boundary acoustic wave device may further include an impedancematching circuit in the interface or on the outer surface of the firstor the second medium layer.

The second medium layer may have a thickness of about 0.5λ or more andthe sound absorbing layer may have a thickness of about 1.0λ or more.

Preferably, a sound absorbing layer of the boundary acoustic wave deviceaccording to a preferred embodiment of the present invention has amultilayer structure.

The sound absorbing layer may have a multilayer structure including aplurality of sound absorbing material layers. A sound absorbing materiallayer close to the second medium layer has an acoustic characteristicimpedance between the acoustic impedances of the second medium layer anda sound absorbing material layer farther from the second medium layer.

The boundary acoustic wave device may further include a mounting boardbonded to a mounting surface with a bump, made of a material harder thanthe structure including the first and second medium layers and the soundabsorbing layer. The boundary acoustic wave device is mounted using themounting board.

The boundary acoustic wave device may further include a stress absorberon a surface of the mounting side.

Preferred embodiments of the present invention are also directed to amethod for manufacturing a boundary acoustic wave device. The methodincludes the steps of forming an electrode on a first medium layer,forming a second medium layer so as to cover the electrode, and forminga sound absorbing layer on one or both of the surfaces of the firstmedium layer and/or the second medium layer opposite the interfacetherebetween.

The step of forming the sound absorbing layer may include the step ofremoving the gas contained in the sound absorbing layer.

The method may be performed in a mother state in which a plurality ofboundary acoustic wave devices are continuously connected and not yetdivided into individual boundary acoustic wave devices, and the motherstate is divided into boundary acoustic wave devices after the soundabsorbing layer is formed.

Alternatively, the steps before the step of forming the sound absorbinglayer may be performed in a mother state, and the step of forming thesound absorbing layer is performed after the mother state is dividedinto boundary acoustic wave devices.

In the boundary acoustic wave device of preferred embodiments of thepresent invention, the electrode is disposed in the interface betweenthe first medium layer and the second medium layer, and a soundabsorbing layer for attenuating modes producing spurious responses isfurther provided on the outer surface of the first and/or the secondmedium layer opposite to the interface. The sound absorbing layerprevents a plurality of spurious responses in a higher region than theresonant frequency or the pass band effectively, as shown in theexperiments described below. Consequently, the resulting boundaryacoustic wave device can exhibit superior resonance characteristics orfilter characteristics.

If the acoustic velocity of the transverse waves in the sound absorbinglayer is lower than that of the transverse waves in the first and/orsecond medium layer having the sound absorbing layer, undesired spuriousresponses of the transverse waves can be reduced effectively.

If the acoustic velocity of the longitudinal waves in the soundabsorbing layer is lower than that of the longitudinal waves in thefirst and/or second medium layer having the sound absorbing layer,undesired spurious responses of the longitudinal waves can be reducedeffectively.

In particular, if the acoustic velocity of the transverse waves in thesound absorbing layer is in the range of about 0.13 to about 1.23 timesthe acoustic velocity of the transverse waves in the first and/or secondmedium layer having the sound absorbing layer, undesired spuriousresponses of the transverse waves can be reduced effectively.

If the acoustic impedance of the sound absorbing layer is in the rangeof about 0.2 to about 5.3 times the acoustic impedance of the firstand/or second medium layer, a plurality of undesired spurious responsescan be reduced effectively.

If the sound absorbing layer is made of the same type of material as thefirst and/or the second medium layer, the sound absorbing layer can beformed in a similar step of forming the first and/or the second mediumlayer.

If a low attenuation constant layer having a lower attenuation constantfor the boundary acoustic waves than the sound absorbing layer isprovided outside the sound absorbing layer, the moisture resistance canbe enhanced because many of the films having a low attenuation constantare superior in compactness. Therefore, the sound absorbing layer andits underlying layers can be protected effectively.

The sound absorbing layer may be made of a variety of materials. A soundabsorbing layer made of at least one material including resin, glass,ceramic, and metal can sufficiently absorb sound and is relatively hard.Accordingly, such a sound absorbing layer can reduce undesired spuriousresponses effectively and lead to a boundary acoustic wave device havinga superior strength. The sound absorbing layer is not necessarily madeof a single material. Many resin materials have high attenuationconstants and they can provide sound absorbing media having variousacoustic velocities or acoustic characteristic impedances by adding afiller of a ceramic or a metal, such as carbon, silica, and tungsten.For example, a resin material, such as epoxy resin, containing thefiller not only facilitates the control of the acoustic velocity or theacoustic characteristic impedance, but also scatters acoustic waves toincrease the attenuation constant.

If the sound absorbing layer is disposed on the surface of the firstand/or the second medium layer so as to oppose the boundary acousticwave propagation path in the interface, the sound absorbing layer canreduce undesired spurious responses effectively.

If an electrically conductive layer is provided on at least one surfaceof the sound absorbing layer, the electrically conductive layer canserve as an electromagnetic shield.

If the boundary acoustic wave device further includes a through-holeelectrode passing through the first medium layer and/or the secondmedium layer, electrically connected to the electrode disposed at theinterface, and an external electrode disposed on an external surface ofthe boundary acoustic wave device, connected to the through-holeelectrode, the boundary acoustic wave device can be electricallyconnected using the through-hole electrode. Consequently, the size ofthe boundary acoustic wave device can be minimized.

A through-hole electrode filled with an elastic material withoutcavities can reduce the difference in acoustic impedance from the mediumlayers. Consequently, undesired reflection and scattering of theboundary acoustic waves can be prevented. Furthermore, the penetrationof corrosive gases can be prevented.

If the through-hole electrode is provided separately in the first mediumlayer and the second medium layer and the through-hole electrodes of thefirst medium layer and the second medium layer do not overlap in thethickness direction, the penetration of corrosive gases can beprevented.

If a wiring electrode is further provided on an external surface of theboundary acoustic wave device and electrically connected to theelectrode disposed at the interface, the electrode in the boundaryacoustic wave device can be extended to the external surface.

If the boundary acoustic wave device has steps on a side surfaceintersecting the interface, and the wiring electrode on the externalsurface is connected to a connection electrode at the steps, thereliability of the electrical connection can be enhanced.

If a third material layer having a lower linear expansion coefficient inthe direction parallel to the interface than the first and the secondmedium layer is provided in at least one of regions between the firstmedium layer and the second medium layer, on the outer surface of thefirst medium layer, and on the outer surface of the second medium layer,external deformation resulting from temperature changes, such as a warp,can be prevented. In addition, the temperature dependency of thefrequency characteristics can be improved, such as the center frequencyof a filter or the resonant frequency of a resonator.

If a third material layer having a linear expansion coefficient in thedirection parallel to the interface, with the opposite sign to that ofthe first and the second medium layer is provided in at least one ofregions between the first medium layer and the second medium layer, onthe outer surface of the first medium layer, and on the outer surface ofthe second medium layer, external deformation resulting from temperaturechanges, such as a warp, can be further prevented. In addition, thetemperature dependency of the frequency characteristics can be improved,such as the center frequency of a filter or the resonant frequency of aresonator.

If a fourth material layer having a higher thermal conductivity than thefirst and the second medium layer is provided in at least one of regionsbetween the first medium layer and the second medium layer, on the outersurface of the first medium layer, and on the outer surface of thesecond medium layer, the boundary acoustic wave device can enhance theheat dissipation ability and prevent temperature increase when highpower is applied. Thus, the electrical power resistance can be improved.

If an impedance matching circuit is provided in the interface or on theouter surface of the first or the second medium layer, the boundaryacoustic wave device contains the impedance matching circuit.

If the second medium layer has a thickness of about 0.5λ or more and thesound absorbing layer has a thickness of about 1.0λ or more, undesiredspurious responses can be reduced more effectively according to anotherpreferred embodiment of the present invention.

If the sound absorbing layer has a multilayer structure, desiredcharacteristics can be easily given to the sound absorbing multilayer byselecting the thickness and the material of each layer of the soundabsorbing multilayer.

If the sound absorbing layer has a multilayer structure including aplurality of material layers and a sound absorbing material layer closeto the second medium layer has an acoustic characteristic impedancebetween the acoustic impedances of the second medium layer and a soundabsorbing material layer farther away from the second medium layer, theacoustic impedances of the second medium layer and the outer soundabsorbing material layer can be highly matched.

If the boundary acoustic wave device has a mounting board bonded to themounting surface with a bump and the mounting board is made of amaterial that is harder than the structure including the first andsecond medium layers and the sound absorbing layer, the boundaryacoustic wave device can be easily mounted on a printed board or thelike using the mounting board. Since the mounting board has a relativelyhigh strength, stress from the printed board or the like can beprevented from being transmitted to the boundary acoustic wave chip sideeven if the mounting on the printed board is made by soldering.Consequently, even if the printed board is warped for example, theboundary acoustic wave device can be prevented from deteriorating infrequency characteristics, or from cracking.

If a stress absorber is provided between the boundary acoustic wavedevice and the mounting board, the stress absorber prevents stresscaused by the warp or the like of the printed board, to which a mountingstructure is to be fixed, from being transmitted to the boundary wavechip. Consequently, the boundary wave chip can be prevented fromwarping, deteriorating in frequency characteristics, and cracking.

The method for manufacturing a boundary acoustic wave device accordingto a preferred embodiment of the present invention includes the steps offorming an electrode on a first medium layer, forming a second mediumlayer so as to cover the electrode, and forming a sound absorbing layeron one or both of the surface of the first medium layer and/or thesecond medium layer opposite the interface therebetween, thus providingthe boundary acoustic wave device according to another preferredembodiment of the present invention.

If the step of forming the sound absorbing layer includes the step ofremoving the gas contained in the sound absorbing layer, the changes offrequency characteristics with time can be reduced.

If in the manufacturing method of a preferred embodiment of the presentinvention, the steps up to the step of forming the sound absorbing layerare performed in a mother state in which a plurality of boundaryacoustic wave devices are continuously connected and the mother state isdivided into boundary acoustic wave devices after the sound absorbinglayer is formed, the boundary acoustic wave device of another preferredembodiment of the present invention can be efficiently manufactured.Alternatively, if the steps before the step of forming the soundabsorbing layer are performed in the mother state, and the step offorming the sound absorbing layer is performed after the mother state isdivided into boundary acoustic wave devices, the sound absorbing layercan cover the entire chip except an external terminal, thereby enhancingthe environmental resistance of the boundary wave device.

Other features, elements, steps, advantages and characteristics of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments thereof with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a front sectional view and a schematic perspectiveview, respectively, of a boundary acoustic wave device according to apreferred embodiment of the present invention.

FIG. 2 is a graph of the displacement distribution of boundary acousticwaves in the main mode when a SiO₂/Au/LiNbO₃ structure has an Authickness of about 0.05λ and a SiO₂ film thickness of about 1.5λ.

FIGS. 3A and 3B are graphs of displacement distributions in spuriousmodes under the same conditions as in FIG. 2.

FIGS. 4A and 4B are graphs of displacement distributions in spuriousmodes under the same conditions as in FIG. 2.

FIGS. 5A and 5B are graphs of displacement distributions in spuriousmodes under the same conditions as in FIG. 2.

FIGS. 6A and 6B are graphs of displacement distributions in spuriousmodes under the same conditions as in FIG. 2.

FIG. 7 is a graph of a displacement distribution in another spuriousmode under the same conditions as in FIG. 2.

FIG. 8 is a graph of the impedance-frequency characteristics and thephase-frequency characteristics of the boundary acoustic wave deviceshown in FIG. 1.

FIGS. 9A and 9B are graphs showing the changes in acoustic velocity andattenuation constant of SH boundary waves, Stoneley waves, and variousspurious modes when the density ρ of the sound absorbing layer isvaried.

FIGS. 10A and 10B are graphs showing the changes in acoustic velocityand attenuation constant of SH boundary waves, Stoneley waves, andvarious spurious modes when the transverse acoustic wave velocity Vs ofthe sound absorbing layer is varied.

FIGS. 11A and 11B are graphs showing the changes in acoustic velocityand attenuation constant of SH boundary acoustic waves, Stoneley waves,and various spurious modes when the transverse acoustic wave velocity Vsof the sound absorbing layer is varied under the condition where theacoustic characteristic impedance Zs of the sound absorbing layer isfixed.

FIGS. 12A and 12B are graphs showing the changes in acoustic velocityand attenuation constant of SH boundary waves, Stoneley waves, andvarious spurious modes when the acoustic characteristic impedance Zs ofthe sound absorbing layer is varied under the condition where thetransverse acoustic wave velocity of the sound absorbing layer is fixed.

FIG. 13 is a graph showing the relationship between the transverseacoustic wave velocity ratio and the impedance ratio in spurious modeswhen the thickness of the SiO₂ film is varied.

FIG. 14 is a graph showing the relationship between the acousticimpedance ratio and the impedance ratio in spurious modes when thethickness of the SiO₂ film is varied.

FIG. 15 is a graph showing the impedance-frequency characteristics andthe phase-frequency characteristics of a boundary acoustic waveresonator according to a preferred embodiment of the present invention.

FIG. 16 is a graph showing the attenuation-frequency characteristics ofa boundary acoustic wave filter according to a preferred embodiment ofthe present invention.

FIG. 17 is a fragmentary front sectional view of a boundary acousticwave device according to a modification of a preferred embodiment of thepresent invention.

FIG. 18 is a fragmentary front sectional view of a boundary acousticwave device according to another modification of a preferred embodimentof the present invention.

FIG. 19 is a fragmentary front sectional view of a boundary acousticwave device according to another modification of a preferred embodimentof the present invention.

FIG. 20 is a fragmentary front sectional view of a boundary acousticwave device according to another modification of a preferred embodimentof the present invention.

FIG. 21 is a fragmentary front sectional view of a boundary acousticwave device according to another preferred embodiment of the presentinvention.

FIG. 22 is a fragmentary front sectional view of a boundary acousticwave device according to another preferred embodiment of the presentinvention.

FIG. 23 is a perspective view of the principal parts of the boundaryacoustic wave device shown in FIG. 22.

FIG. 24 is a front sectional view of a boundary acoustic wave deviceaccording to another preferred embodiment of the present invention.

FIG. 25 is a front sectional view of a boundary acoustic wave deviceaccording to another preferred embodiment of the present invention.

FIG. 26 is a front sectional view of a boundary acoustic wave deviceaccording to another preferred embodiment of the present invention.

FIGS. 27A to 27G are front sectional views illustrating a method formanufacturing a boundary acoustic wave device according to a preferredembodiment of the present invention.

FIGS. 28A to 28F are front sectional views illustrating a method formanufacturing a boundary acoustic wave device according to anotherpreferred embodiment of the present invention.

FIGS. 29A to 29H are front sectional views illustrating a method formanufacturing a boundary acoustic wave device according to anotherpreferred embodiment of the present invention.

FIGS. 30A to 30F are front sectional views illustrating a method formanufacturing a boundary acoustic wave device according to anotherpreferred embodiment of the present invention.

FIG. 31 is a fragmentary sectional view illustrating a method formanufacturing a boundary acoustic wave device according to anotherpreferred embodiment of the present invention.

FIG. 32 is a front sectional view of a known boundary acoustic wavedevice.

FIG. 33 is a schematic plane view of the electrode structure of a 1-portboundary acoustic wave resonator prepared as the known boundary acousticwave device.

FIG. 34 is a representation of spurious modes presented in theimpedance-frequency characteristics of a known boundary acoustic wavedevice.

FIG. 35 is a ladder-type circuit including a plurality of known boundaryacoustic wave devices.

FIG. 36 is a graph showing the attenuation-frequency characteristics ofa ladder-type filter constituted of a plurality of known boundaryacoustic wave elements.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be further described in detail by way ofspecific preferred embodiments with reference to the drawings.

The present inventors conducted a numerical analysis to find the causeof the above-described spurious responses. This numerical analysis wasbased on the method disclosed in the literature “A Method For EstimatingOptimal Cuts and Propagation Directions For Excitation and PropagationDirections For Excitation of Piezoelectric Surface Waves” (J. J.Campbell and W. R. Jones, IEEE Trans. Sonics and Ultrasonics, Vol. SU-15(1968), pp. 209-217). In this analysis, the displacements and thevertical stress at the interfaces between SiO₂ and Au and between the Auand LiNbO₃ were continuous, and the potential was 0 by theshort-circuited interfaces. The SiO₂ had a predetermined thickness andthe thickness of the 15° Y-X propagating LiNbO₃ was unlimited. Thedisplacement distribution of the boundary waves and spurious modes werethus examined.

FIG. 2 shows the displacement distribution of boundary acoustic waves inthe main mode when the Au thickness was 0.05λ and the SiO₂ thickness was1.5λ.

FIGS. 3A to 7 show displacement distributions of spurious modes underthe same conditions as in FIG. 2. The displacement distribution in FIG.2 and the displacement distributions of the spurious modes in FIGS. 3Ato 7 are shown in ascending order of the response frequencies of theirrespective boundary waves and spurious modes. Specifically, the boundarywaves of FIG. 2 have the lowest frequency and the spurious mode waves ofFIG. 7 have the highest frequency. λ represents the wavelength of theboundary waves in the main mode.

In FIGS. 2 to 7, the solid line represents component U1 (component alongthe X₁ direction in displacement), the broken line represents componentU2 (component along the X₂ direction in the displacement), and thedotted-chain line represents component U3 (component along the X₃direction in the displacement).

The X₁ direction refers to the direction in which boundary wavespropagate at the interface; the X₂ direction refers to the directionperpendicular to the X₁ direction in the plane of the interface; the X₃direction refers to the direction perpendicular to the interface.

In general, acoustic waves include p waves defined by U1, SH wavesdefined by component U2, and SV waves defined by component U3. Theboundary acoustic waves and the spurious modes are each in a modeaccording to a combination of these partial waves, P, SH, and SV waves.In FIGS. 3B to 7, only the Au layer is shown and the SiO₂ layer and theLiNbO₃ layer are omitted in the figures for the sake of simplification.

FIG. 2 clearly shows that the primary component of the boundary waves,or the main mode, is the SH acoustic waves essentially composed ofcomponent U2.

As is clear from FIGS. 3A to 7, spurious modes are roughly divided intothree types: one essentially composed of component U2; anotheressentially composed of components U1 and U3; and the other essentiallycomposed of component U1. Waves in these three types of spurious modespropagate with most of their energy confined between the surface of theSiO₂ second medium layer and the Au electrode provided in the interface.The occurrence of the three types of spurious modes produces theabove-mentioned plurality of spurious responses shown in FIGS. 34 and36.

In addition to the spurious modes shown in FIGS. 3A to 7, another modesimilar to the Stoneley waves essentially composed of components U1 andU3 (hereinafter collectively referred to as Stoneley waves) propagatesas well. However, the SiO₂/Au (0.05%)/15° Y-X propagating LiNbO₃structure has an electromechanical coupling coefficient of about 0 forthe Stoneley waves, and accordingly does not excite Stoneley waves.

In consideration of the results shown in FIGS. 2 to 7, the presentinventors thought that the above-mentioned plural spurious responses inthe higher region can be reduced by controlling the modes in which wavespropagate with their energy confined in the second medium layer, andconsequently discovered and developed the present invention.

FIGS. 1A and 1B are a front sectional view and a schematic perspectiveview of a boundary acoustic wave device according to a first preferredembodiment of the present invention, respectively.

The boundary acoustic wave device 1 includes a first medium layer 2. Thefirst medium layer 2 is preferably formed of a 15° Y-X propagatingLiNbO₃ single crystal substrate.

However, the first medium layer 2 may be made of any other singlecrystal substrate. For example, a LiNbO₃ piezoelectric single crystalsubstrate may be used or other piezoelectric single crystals, such asLiTaO₃, may be used.

An IDT 3 and reflectors 4 and 5 are provided on the upper surface of thefirst medium layer 2. In the present preferred embodiment, gratingreflectors 4 and 5 are disposed at both sides of the IDT 3 to form a1-port boundary acoustic wave element.

A second medium layer 6 is arranged to cover the IDT 3 and thereflectors 4 and 5. The second medium layer 6 is made of a SiO₂ film inthe present preferred embodiment.

Further, a sound absorbing layer 7 is provided on the upper surface ofthe second medium layer 6. The sound absorbing layer 7 is made of aresin, for example, having an acoustic wave attenuation constant higherthan that of the second medium layer 6. In the present preferredembodiment, the IDT 3 and the reflectors 4 and 5 are formed bydepositing an Au main electrode layer to a thickness of about 0.05λ on aabout 0.003λ thick NiCr contact layer. The IDT preferably has 50 pairsof electrode fingers having a duty ratio of about 0.55 with weighting byvarying the finger overlap, and opposing bus bars at an interval ofabout 30.5λ, as in the boundary acoustic wave element that exhibited thecharacteristics shown in FIG. 34. The numbers of the electrode fingersof the reflectors 4 and 5 are preferably about 50. The intervals betweenthe ends of the electrode fingers of the IDT are preferably about 0.25λand the maximum finger overlap is preferably about 30λ. The IDT 3 andthe reflectors 4 and 5 preferably have the same λ value, and the centerdistance between the electrode fingers of the IDT 3 and the reflectors 4and 5 is preferably about 0.5λ. The SiO₂ film preferably has a thicknessof about 2λ. Under these conditions, deposition is performed by RFmagnetron sputtering at wafer heating temperature of about 200° C. inthe same manner as in the example of the characteristic comparison shownin FIG. 34.

Hence, the boundary acoustic wave device 1 has the same structure as theabove-described comparative example, except for having the soundabsorbing layer 7.

The sound absorbing layer 7 is made of an epoxy resin having acontrolled hardness and has a thickness of about 5λ or more. The soundabsorbing layer 7 is formed by applying the epoxy resin onto the secondmedium layer 6 and curing the resin.

FIG. 8 shows the impedance-frequency characteristics and thephase-frequency characteristics of the boundary acoustic wave device 1.

As is clear from the comparison of FIGS. 8 and 34, while the impedanceratio (ratio of impedance at the resonant frequency to that at theantiresonant frequency) of the spurious response around 1700 MHz is 29.3dB in FIG. 34, it is remarkably reduced to about 7.1 dB in the presentpreferred embodiment. Hence, undesired spurious responses in the highfrequency region can be reduced effectively by providing the soundabsorbing layer 7.

However, the spurious response around 1700 MHz is not completelyeliminated in the frequency characteristics shown in FIG. 8.

The present inventors have discovered that when acoustic waves propagatethrough a multilayer composite including a layer with a low acousticvelocity and a layer with a high acoustic velocity, the acoustic wavespropagate with their energy concentrated on the low-acoustic-velocitylayer. Accordingly, the sound absorbing layer 7 on the surface of thesecond medium layer is formed of a material having a low acoustic wavevelocity and thus, a multilayer composite of sound absorbing layer7/second medium layer/electrode/first medium layer 2 is formed, so thatenergy in modes producing spurious responses is transferred from thesecond medium layer to the sound absorbing layer 7.

More specifically, the sound absorbing layer 7 serves as a soundabsorbing medium, and the energy of the spurious modes transferred tothe sound absorbing layer 7 does not return to the second medium layer6. In this instance, the boundary acoustic waves, which are mainresponses of the boundary acoustic wave device 1, propagate with theirenergy concentrated on the vicinity of the interface. Therefore, theenergy of the boundary acoustic waves is difficult to reduce.

In the analyses shown in FIGS. 2 to 7, three types of spurious modeswere found: one is essentially composed of SV waves; another isessentially composed of SH waves; and the other is essentially composedof P waves.

Hence, in order to highly reduce spurious modes essentially composed ofSH waves or SV waves, the transverse acoustic wave velocity in the soundabsorbing layer 7 can be lower than the transverse acoustic wavevelocity in the second medium layer. In order to reduce spurious modesessentially composed of P waves effectively, the longitudinal acousticwave velocity in the sound absorbing layer 7 can be lower than thelongitudinal acoustic wave velocity in the second medium layer.

The energy T of a mode transferring from the second medium layer 6 tothe sound absorbing layer 7 is expressed by T=4Z₀Z_(L)/(Z₀+Z_(L))²,wherein Z₀ represents the acoustic characteristic impedance of thesecond medium layer 6, and Z_(L) represents the acoustic characteristicimpedance of the sound absorbing layer 7.

As is clear from the above-described equation, the closer the acousticcharacteristic impedance Z₀ of the second medium layer 6 to the acousticcharacteristic impedance Z_(L) of the sound absorbing layer 7, thehigher the energy T transferring from the second medium layer 6 to thesound absorbing layer 7. Thus, spurious modes can be efficientlyreduced.

Accordingly, in order to reduce the spurious modes essentially composedof SH waves or SV waves, it is preferable that the acousticcharacteristic impedances of the second medium layer 6 and the soundabsorbing layer 7 for transverse waves be matched to each other, thatis, be brought close to each other. Also, in order to reduce thespurious modes essentially composed of P waves, it is preferable thatthe acoustic impedances of the second medium layer 6 and the soundabsorbing layer 7 for longitudinal waves be matched to each other.

Therefore, the sound absorbing layer 7 is preferably made of a materialhaving a lower acoustic velocity than the second medium layer, highacoustic matching with the second medium layer, and a high soundabsorbing effect.

In order to confirm this conclusion, an analysis was conducted using theboundary acoustic wave device 1 shown in FIG. 1. In this analysis, thefirst medium layer 2 was made of a 15° Y-X LiNbO₃ substrate with aninfinite thickness, and the second medium layer 6 was made of a 1.5λthick SiO₂ film. The IDT was made of a 0.05λ thick Au film. Thedisplacement and the vertical stress at the interfaces between the soundabsorbing layer 7 and the SiO₂ film, between the SiO₂ film and the Au,and between the Au and the LiNbO₃ were continuous, and the potential was0 by the short-circuited interfaces between the SiO₂ and Au and betweenthe Au and the LiNbO₃. The sound absorbing layer 7 was considered to beisotropic and of infinite thickness. Thus, the manner was simulated inwhich waves passing from the SiO₂ film to the sound absorbing layer 7are absorbed. The acoustic velocity and the propagation loss of boundarywaves and spurious mode waves were obtained using this structure.

The acoustic velocity of and the acoustic characteristic impedance forlongitudinal waves and transverse waves propagating through an isotropicmaterial will now be described. When the acoustic velocity of transversewaves is Vs, the acoustic velocity of longitudinal waves is Vp, and theacoustic characteristic impedance for the transverse waves is Zs, andthe acoustic characteristic impedance for the longitudinal waves is Zp;Vs and Vp are expressed by the following equations with the elasticstiffness coefficients C11 and C12 and the density ρ of the isotropicmaterial.

$\begin{matrix}{{{Vs} = \sqrt{\frac{{C\; 11} - {C\; 12}}{2\rho}}}{{Vp} = \sqrt{\frac{C\; 11}{\rho}}}{{Zs} = {\rho \; {Vs}}}{{Zp} = {\rho \; {Vp}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

FIGS. 9A to 10B show the relationships between the density ρ of thesound absorbing layer of the boundary acoustic wave device and theacoustic velocity of propagating boundary waves, a spurious mode mainlycontaining component U2, and a spurious mode mainly containing componentU3, and the relationships between the transverse acoustic wave velocityVs of the sound absorbing layer and the attenuation constant in thesemodes. The boundary acoustic wave device had the same structure as inabove, except that the density ρ of the sound absorbing layer wasvaried.

In FIG. 9A to below-described FIG. 12B, the spurious modes mainlycontaining component U2 include a high U2-1 mode in which component U2is of a lower degree, and a high U2-2 mode in which component U2 is of ahigher degree. The spurious modes mainly containing component U3 includea high U3-1 mode in which component U3 is of a lower degree, and a highU3-2 mode in which component U3 is of a higher degree.

SiO₂ has a transverse acoustic wave velocity of 3757 m/s and a densityof 2210 kg/m³. Accordingly, when the sound absorbing layer has atransverse acoustic wave velocity of 3757 m/s and a density of 2210kg/m³, the attenuation constant in spurious modes becomes the maximumand spurious responses are suppressed. In contrast, SH boundary wavesand Stoneley waves are not attenuated at all. In the structure used forthe calculation, the electromechanical coupling coefficient of Stoneleywaves is about 0, as mentioned above. As a result, Stoneley waves canpropagate, but do not produce spurious responses because they are notexcited.

Since, in the structure used for FIGS. 9A to 10B, only the density ofthe sound absorbing layer was varied, the effects of the transverseacoustic wave velocity and the acoustic characteristic impedance for thetransverse waves in the sound absorbing layer cannot be estimated.Accordingly, the acoustic characteristic impedance Zs for transversewaves in the sound absorbing layer was a value equal to the acousticcharacteristic impedance of about 8.30×10⁶ kg/m²·s of SiO₂ fortransverse waves, and a constant was determined from the equation below.Then the relationships between the acoustic velocity and the attenuationconstant for SH boundary acoustic waves, spurious mode waves mainlycontaining component U2, and spurious mode waves mainly containingcomponent U3 were obtained by varying the transverse acoustic wavevelocity Vs of the sound absorbing layer. The results are shown in FIGS.11A and 11B.

C11=C12+2Zs ²/ρ

Then, the transverse acoustic wave velocity Vs in the sound absorbinglayer was about 3,757 m/s and a constant was determined from theequation below. The relationship between the acoustic velocity and theattenuation constant for SH boundary waves, spurious mode waves mainlycontaining component U2, and spurious mode waves mainly containingcomponent U3 were obtained by varying the acoustic characteristicimpedance Zs. The results are shown in FIGS. 12A and 12B.

C11=C12+2ρVs ²

FIGS. 12A and 12B show that the closer is the transverse acoustic wavevelocity Vs in the sound absorbing layer to 3757 m/s, or the acousticvelocity of SiO₂, the larger are the attenuation constants in spuriousmodes, and that when the velocity of acoustic waves is lower than theacoustic velocity of SiO₂, the attenuation constant is increased. An IDTused in a boundary acoustic wave device generally has about 10 to about50 pairs of electrode fingers, and the range of the aperture is about10λ to about 50λ relative to the propagation wavelength. The calculatedattenuation constant α represents energy radiation in the ±X₃directions. When the attenuation constant in a spurious mode is about0.5 dB/λ, an IDT of 10λ exhibits an attenuation of, for example, about 5dB and an IDT of about 50λ exhibits an attenuation of, for example,about 25 dB. These attenuations result from the radiation of acousticwaves to the sound absorbing layer.

In addition, the sound absorbing effect of the sound absorbing layerincreases the attenuation. Thus, spurious responses can be sufficientlysuppressed. If the attenuation constant in spurious modes is at leastabout 1.0 dB/λ to 1.5 dB/λ, spurious responses can further be minimized.

It was shown that, when the transverse acoustic wave velocity Vs in thesound absorbing layer was preferably about 0.13 to about 1.23 times ashigh as the transverse acoustic wave velocity in the SiO₂, spuriousmodes mainly containing a higher degree of component U2 or U3, or highU2-2 modes and high U3-2 modes, were attenuated by about 1.5 dB/λ ormore, and a spurious mode mainly containing a lower degree of componentU2, or a high U2-1 mode, was attenuated by about 0.5 dB/λ or more. Morepreferred transverse acoustic wave velocity Vs in the sound absorbinglayer is about 0.6 to about 1.00 times the transverse acoustic wavevelocity in the SiO₂. In this case, a spurious mode mainly containing alower degree of component U3, or high U3 mode, is attenuated by about0.5 dB or more. In modes mainly containing the same type of partialwaves, as the degree is reduced, the electromechanical couplingcoefficient tends to increase and spurious responses tend to be larger.

The spurious mode denoted by high U2-2 which mainly contains the secondhigher degree of component U2 has an attenuation constant of about 0.003dB/λ, even if the transverse acoustic wave velocity Vs in the soundabsorbing layer is 5,000 m/s or more. Also, the spurious mode mainlycontaining the second higher degree of component U3 (high U3-2) has anattenuation constant of about 0.477 dB/λ, even if transverse acousticwave velocity Vs in the sound absorbing layer is 5,000 m/s or more.These attenuation constants result from energy radiation toward thesingle crystal substrate that occurs due to higher acoustic velocitiesof the spurious modes than the acoustic velocity of SH waves or SV wavesin the LiNbO₃ single crystal substrate.

FIGS. 12A and 12B also show that the closer is the acousticcharacteristic impedance Zs of the sound absorbing layer to the acousticcharacteristic impedance of about 8.30×10⁶ kg/m²·s of SiO₂, the higherare the attenuation constants in spurious modes. The spurious modedenoted by high U2-1 is attenuated by about 0.5 dB/λ or more when theacoustic characteristic impedance Zs of the sound absorbing layer isabout 0.45 to about 3.61 times the acoustic characteristic impedance ofSiO₂; it is favorably attenuated by about 1.0 dB/λ or more when Zs isabout 0.75 to about 1.99 times the acoustic characteristic impedance; itis more favorably attenuated by about 1.5 dB/λ or more when Zs is about0.89 to about 1.48 times the acoustic characteristic impedance.

The spurious mode denoted by high U2-2 is attenuated by about 0.5 dB/λor more when the acoustic characteristic impedance of the soundabsorbing layer is about 0.20 to about 5.30 times the acousticcharacteristic impedance of SiO₂, favorably attenuated by about 1.0 dBor more when it is about 0.41 to about 3.25 times the acousticcharacteristic impedance, and more favorably attenuated by about 1.5dB/λ or more when it is about 0.57 to about 1.88 times the acousticcharacteristic impedance.

The spurious mode denoted by high U3-1 is attenuated by about 0.5 dB/λor more when the acoustic characteristic impedance Zs of the soundabsorbing layer is about 0.84 to about 1.29 times the acousticcharacteristic impedance of SiO₂, favorably attenuated by about 1.0 dB/λor more when it is about 0.96 to about 1.08 times the acousticcharacteristic impedance, and more favorably attenuated by about 1.5 dBor more when it is about 0.99 to about 1.02 times the acousticcharacteristic impedance.

The spurious mode denoted by high U3-2 is attenuated by about 0.5 dB/λor more when the acoustic characteristic impedance Zs of the soundabsorbing layer is about 0.71 times or more the acoustic characteristicimpedance of SiO₂, favorably attenuated by about 1.0 dB/λ or more whenit is about 0.76 to about 1.98 times the acoustic characteristicimpedance, and more favorably attenuated by about 1.5 dB/λ or more whenit is about 0.89 to about 1.47 times the acoustic characteristicimpedance.

In order to prove the calculations, experiments were conducted. Boundaryacoustic wave devices 1 according to the preferred embodiment shown inFIG. 1 were prepared by forming a variety of sound absorbing layerstightly on the surface of the SiO₂ of the boundary acoustic waveresonator having the characteristics shown in FIG. 34, prepared as acomparative example. FIG. 13 shows the relationship between thetransverse acoustic wave velocity ratio (Vs ratio) and the impedanceratio in spurious modes. The Vs ratio is obtained by dividing thetransverse acoustic wave velocity of the sound absorbing layer by thetransverse acoustic wave velocity of SiO₂. FIG. 14 shows therelationship between the acoustic impedance ratio (Zs ratio) and theimpedance ratio in spurious modes. The Zs ratio is obtained by dividingthe acoustic impedance of the sound absorbing layer for transverse wavesby the acoustic impedance of SiO₂ for the transverse waves. Theimpedance ratio in spurious modes refers to the fact that in a spuriousmode where the ratio of the impedance at the resonant frequency to theimpedance at the antiresonant frequency is highest.

The above-described boundary acoustic wave device has an impedance ratioin spurious modes of about 7.1 dB or more when the Vs ratio is about0.273 and the Zs ratio is about 0.127. However, if the Zs ratio of thesound absorbing layer is about 0.393 or more, the impedance ratio inspurious modes decreases to about 3.9 dB or less, and thus the impedanceratio in spurious modes decreases as the Zs ratio becomes closer to 1.If the Vs ratio is about 0.488 or more, the impedance ratio in spuriousmodes decreases to about 3.9 dB or less, and thus the impedance ratio inspurious modes decreases as the Vs ratio becomes closer to 1.

FIG. 15 shows the resonance characteristics of a boundary acoustic waveresonator having a sound absorbing layer with a Vs ratio of about 0.633and a Zs ratio of about 0.547 on the surface of the SiO₂ film. FIG. 16shows the filter characteristics of a ladder-type filter using theboundary acoustic wave resonators.

While the attenuation constant for transverse waves was about 7.1 dB/λin FIG. 8 when the sound absorbing layer produced the sound absorbingeffect, the structure having the characteristics shown in FIGS. 15 and16 showed that the attenuation constant for transverse waves of thesound absorbing layer was about 1.75 dB/λ. These results suggest that asound absorbing layer simply having a large attenuation constant cannotsufficiently suppress spurious modes, and that the spurious modes can besuppressed more effectively by matching the acoustic impedance.

In a preferred embodiment of the present invention, the sound absorbinglayer may be made of the same type of material as the second mediumlayer. Even in such a case, the attenuation constant can be reduced onlyin the region where the energy of boundary acoustic waves, or in themain response, are present, while being increased in its externalregion. The above-mentioned “same type of material” does not necessarilymean one and the same material. For example, if SiO₂ is used, two SiO₂films may have different characteristics depending on the depositionmethod, as described below. The same type of material can be acombination of two different SiO₂ films. In the deposition of the secondmedium layer by sputtering, in general, low-quality materials havinghigh attenuation constants can be deposited at a high speed and areinexpensive, while high-quality materials having low attenuationconstants are deposited at a low speed and are expensive. For example, aboundary acoustic wave device having a SiO₂ film/Al electrode/Auelectrode/LiNbO₃ structure may be provided with a second SiO₂ filmserving as the sound absorbing layer over that SiO₂ film. In thisinstance, the SiO₂ film serving as the second medium layer can be formedof a high-quality SiO₂ film having a low attenuation constant to athickness of, for example, about 0.5λ, and the second SiO₂ film servingas the sound absorbing layer can be formed of a low-quality SiO₂ filmhaving a high attenuation constant to a thickness of, for example, about1.0λ. This structure can facilitate the suppression of spuriousresponses at a low cost almost without degrading the characteristics ofthe boundary acoustic wave device. In this instance, the high-qualitySiO₂ film has an elastic constant and a density close to those of thelow-quality SiO₂ film, and accordingly the displacement distribution ofthe boundary acoustic waves in the main mode hardly changes in the depthdirection. Although the low-quality film and the high-quality film canbe continuously formed with the same apparatus, the low-quality film maybe formed in a below-described process. Specifically, either thehigh-quality film or the low-quality film may be formed by one ofsputtering, spin coating, screen printing, and CVD, and the other filmmay be formed by any one of the other methods.

The first and the second medium layer are not necessarily formed by thesame first material layer. For example, the second medium layer may havea multilayer structure composed of a plurality of medium materiallayers. FIG. 17 is a fragmentary front sectional view of a modificationof the boundary acoustic wave device whose second medium layer has amultilayer structure. In this boundary acoustic wave device 21, an IDT23 is provided on a first medium layer 22, and the IDT 23 is coveredwith a second medium layer 26. The second medium layer 26 has astructure formed by depositing a medium material layer 26 b on anothermedium material layer 26 a. In addition, a sound absorbing layer 27 isprovided on the second medium layer 26.

The medium material layers 26 a and 26 b are each made of an appropriatematerial. For example, the medium material layer 26 a is preferably madeof SiO₂ and the other medium material layer is preferably made of SiN.The second medium layer may be formed by depositing three or morelayers.

The multilayer structure may be constituted of a medium material layerhaving a high attenuation constant and a medium material layer having alow attenuation constant. The high attenuation constant medium materiallayer and the low attenuation constant medium material layer may bealternately deposited. Many of the medium material layers having lowattenuation constants are superior in compactness. By providing such amedium material layer at the outer side from the interface, the moistureresistance around the interface can be enhanced.

In a preferred embodiment of the present invention, the first mediumlayer may have a multilayer structure as well. One of the uniquecharacteristics of a preferred embodiment of the present invention isthat spurious modes are minimized by reducing the attenuation constantof the boundary waves-propagating material at the interface whereboundary waves propagate and in its vicinity, and by providing the soundabsorbing layer on at least a portion of an outer layer. A preferredembodiment of the present invention is also characterized in that theacoustic characteristic impedance of the sound absorbing layer ismatched to that of the boundary wave propagating medium layer asdescribed above to enhance the sound absorbing efficiency of the soundabsorbing layer, and that the acoustic velocity in the sound absorbinglayer is reduced so that spurious modes turn to leaking modes, therebysuppressing spurious responses effectively. It is therefore preferablethat the attenuation constant of the sound absorbing layer for acousticwaves be higher than that of the first and the second medium layer.

The material of the sound absorbing layer used in a preferred embodimentof the present invention is not particularly limited as long as itsattenuation constant for acoustic waves is larger than that of the firstand the second medium layer. Examples of the materials used for formingthe sound absorbing layer include resins, such as epoxy, phenol,acrylate, polyester, silicone, urethane, and polyimide; glasses, such aslow-melting-point glass and water glass; alumina ceramics; and metals.

In particular, many resin materials have high attenuation constants andtheir compositions can be easily controlled, and accordingly resinmaterial can form sound absorbing layers having a variety of acousticvelocities and acoustic characteristic impedances. Thus, the soundabsorbing layer is preferably made of a resin material.

The sound absorbing layer may have a multilayer structure formed bydepositing a plurality of sound absorbing material layers. For example,the boundary acoustic wave device 31 of the modification shown in FIG.18 has a sound absorbing layer 7 formed by depositing a first soundabsorbing material layer 7 a and a second sound absorbing material layer7 b, on the upper surface of the second medium layer 6. In thisinstance, the acoustic characteristic impedance of the sound absorbingmaterial layer 7 a is preferably between the acoustic characteristicimpedances of the second medium layer 6 and the second sound absorbingmaterial layer 7 b, so that the matching of acoustic characteristicimpedances can be enhanced. Since the sound absorbing material layer 7 ain this instance is intended to ensure the acoustic matching between thesecond medium layer 6 and the sound absorbing material layer 7 b, thesound absorbing material layer 7 a preferably has a higher attenuationconstant than the medium layer 6. However, the attenuation constant isnot always required to be higher than that of the medium layer 6. InFIG. 18, description of previously discussed elements and referencenumerals is omitted here for simplicity.

The boundary acoustic wave device of the present preferred embodimentmay have an electrically conductive layer 41 under the sound absorbinglayer 7, as shown in FIG. 19. Also, an electrically conductive layer 42may be provided on the upper surface of the sound absorbing layer 7, asshown in FIG. 20. Thus, in the present preferred embodiment, anelectrically conductive layer can be provided on at least either theupper or the lower surface of the sound absorbing layer, therebypreventing the degradation of attenuation, which results from, forexample, directly transmitted electromagnetic waves between the inputterminal and the output terminal in a filter. Preferably, theelectrically conductive layers 41 and 42 are provided in regionsopposing the region having the IDT and the reflectors with the mediumlayers therebetween, thereby preventing the degradation of attenuationeffectively. In FIGS. 19 and 20, description of previously discussedelements and reference numerals is omitted here for simplicity.

If the IDT includes an input IDT and an output IDT, it is preferablethat the electrically conductive layer be divided into a portionopposing the input IDT with a medium layer therebetween and a portionopposing the output IDT with the medium layer therebetween, and that theportions are grounded with their respective wiring electrodes. Thedegradation of attenuation thus can be prevented more effectively.

Preferably, the sound absorbing layer 7 of the boundary acoustic wavedevice according to a preferred embodiment of the present invention ismade of a resin material, such as resin adhesive. If a gas remains inthe resin material, however, the sound absorbing layer can be crackedduring reflow soldering, or the gas comes out over time to vary thestress on the chip, so that the frequency characteristics can be changeddisadvantageously. In order to prevent these problems, the gas ispreferably removed in a vacuum in the step of forming the soundabsorbing layer of a resin material after the sound absorbing layer isapplied at, for example, room temperature. In this instance, the soundabsorbing material layer can be cured by heating in a vacuum.

In the boundary acoustic wave device of the present preferred embodimentof the present invention, a wiring electrode may be provided for wiringon the upper surface of the second medium layer or the sound absorbinglayer. For example, a boundary acoustic wave device 51 according to themodification shown in FIG. 21 has a wiring electrode 52 on the uppersurface of the sound absorbing layer 7. An end of the wiring electrode52 is electrically connected to the IDT 3 through a through-holeelectrode 53, a through-hole electrode 54, and another wiring electrode55. In this modification, the through-hole electrode 53 is provided inthe sound absorbing layer 7, and the through-hole electrode 54 isprovided in the second medium layer 6.

For example, if the first medium layer 2 is made of a LiNbO₃ substrateand the second medium layer 6 is made of SiO₂, the dielectric constantof the first medium layer 2 is relatively high and the dielectricconstant of the second medium layer 6 is relatively low. In this case,the wiring electrode 52 or the like is preferably provided on the uppersurface of the second medium layer 6 for electrically connecting theabove-described types of electrodes to each other, thereby reducing theparasitic capacitance produced by wiring. For example, if the firstmedium layer 2 is made of a glass substrate, the second medium layer 6is made of a ZnO thin film, and the sound absorbing layer 7 is made of adielectric material with a low dielectric constant, the dielectricconstant of the second medium layer 6 is relatively high and thedielectric constant of the sound absorbing layer 7 is relatively low. Inthis case, the wiring electrode or the like for wiring is preferablyprovided on the upper surface of the sound absorbing layer 7, therebypreventing the parasitic capacitance produced by wiring. The degradationof the filter characteristics or the resonance characteristics of theboundary acoustic wave device thus can be prevented. Parasiticcapacitance reduces the attenuation or reduces the bandwidth of filtersor resonators disadvantageously.

If the wiring electrode 52 is disposed in a different layer from thelayer of the IDT 3, as described above, the connection between theselayers is preferably established by the through-hole electrodes 53 and54. Boundary acoustic waves propagate through the region having the IDT3 while slightly leaking from the electrode. If the through-holeelectrodes 53 and 54, particularly through-hole electrode 54, arehollow, the difference in acoustic impedance between the hollow and themedium layer 6 becomes large, and accordingly the reflection coefficientin the through hole increases. Consequently, boundary acoustic waves canreflect, scatter, or resonate depending on the position of thethrough-hole electrode 54, and spurious responses and attenuation can bereduced disadvantageously. Accordingly, it is preferable that thethrough-hole electrode 54 be filled with an elastic material so that theabove-mentioned difference in acoustic impedance can be reduced.Preferably, the through-hole electrode 53 is filled with an elasticmaterial as well.

The IDT 3, which is generally formed by photolithography, may produce aproblem by resist coating or wafer vacuum suction if the through-holeelectrode 54 is hollow. In view of the prevention of these problems, thehollows of the through-hole electrodes 53 and 54 are preferably filledwith an elastic material. If an electrically conductive material, suchas Cu, is used as the elastic material, the wiring resistance can beadvantageously reduced.

Through-hole electrodes 53 and 54 filled with no elastic material easilyallow gases to flow into the depths of the boundary acoustic wave devicefrom the outside, and consequently, the characteristics of the devicemay be degraded by a corrosive gas. Even in through-hole electrodes 53and 54 sufficiently filled with an elastic material, the difference inthermal expansion or elasticity between the elastic material and thesecond medium layer 6 or other layers of the boundary acoustic wavedevice produces a stress, so that cracks easily occur. Thus, the devicecan be vulnerable to the penetration of corrosive gas from the outside.In particular, if some of the layers of the boundary acoustic wavedevice are formed of an amorphous material such as SiO₂ or apolycrystalline material such as ZnO, cracks as described above causecorrosive gas to penetrate the layers, so that the electrode may becorroded.

In the structure having a plurality of through-hole electrodes 53 and54, as described above, it is preferable that the through-holeelectrodes 53 and 54 not be continued in the thickness direction in theboundary acoustic wave device, as shown in FIG. 21. In FIG. 21, thethrough-hole electrodes 53 and 54 are disposed at different positionswhen viewed from above, and they are connected to each other with theconnection electrode 56. Thus, the penetration of corrosive gas into thedepths of boundary acoustic wave device can be prevented.

The interlaminar wiring using the through-hole electrodes can reduce thechip size of the boundary acoustic wave device because of its highdegree of freedom of wiring. If a single crystal material is used forthe first or the second medium layer or other layers of the boundaryacoustic wave device, however, it is difficult to form through holes.For example, if the medium layers of the boundary acoustic wave devicehave large thicknesses, it is difficult to ensure vertical positions ofthe walls of the through holes or it takes a long time to form thethrough holes. The through holes are generally formed by reactive ionetching with a mixture of Ar and CF₄ gases. In addition, the formationof the holes may degrade the strength of the medium layers, or thechanges in temperature during mounting on a circuit board or in ambienttemperature may cause the chips to crack. Furthermore, boundary acousticwaves may be reflected or scattered at some portions of the throughholes, or a corrosive gas may cause a problem. These problems can besolved by use of the wiring electrode disposed on the external surfaceof the boundary acoustic wave device, instead of use of the through-holeelectrodes.

FIG. 22 is a schematic fragmentary sectional view of a boundary acousticwave device having a wiring electrode at its external surface, accordingto a modification.

The boundary acoustic wave device 61 includes an IDT 63 and reflectors(not shown) on a first medium layer 2. The IDT 63 and the reflectors arecovered with a second medium layer 66. A connection electrode 67 isdisposed between the first medium layer 2 and the second medium layer66, and is connected to the IDT 63. The connection electrode 67 isextended to the external surface of the boundary acoustic wave device61. The second medium layer 66 is provided with a third medium layer 68on its surface. Another wiring electrode 69 is provided between thesecond medium layer 66 and the third medium layer 68. The wiringelectrode 69 is also extended to the external side surface of theboundary acoustic wave device 61.

The first and second medium layers 2 and 66 are provided in the samemanner as the first and second medium layers 2 and 6 of the boundaryacoustic wave device according to the first preferred embodiment. Thethird medium layer 68 is preferably formed of the same material as thesecond medium layer 66. Specifically, in the present preferredembodiment, the multilayer structure including the second medium layer66 and the third medium layer 68 defines the upper medium layeroverlying the interface. The third medium layer 68 however may be formedof a different material than the second medium layer 66.

A sound absorbing layer 7 is provided on the upper surface of the thirdmedium layer 68. The sound absorbing layer 7 is made of the samematerial as the sound absorbing layer 7 in the first preferredembodiment.

A wiring electrode 71 is provided between the third medium layer 68 andthe sound absorbing layer 7. The wiring electrode 71 is also extended tothe external surface of the boundary acoustic wave device 61.

The external side surface of the boundary acoustic wave device 61 isprovided with another wiring electrode 72. The wiring electrode 72electrically connects the connection electrode 67 and the wiringelectrodes 69 and 71 on the external surface of the boundary acousticwave device 61.

In addition, the boundary acoustic wave device 61 has an externalconnection electrode 73 on the upper surface of the sound absorbinglayer 7, and the wiring electrode 72 is connected to the externalconnection electrode 73. Further, in the boundary acoustic wave device61, the external surface of the multilayer structure including thesecond medium layer 66, the third medium layer 68, and the soundabsorbing layer 7, other than the region having the external connectionelectrode 73 is covered with a protective film 74. The protective film74 is appropriately made of an insulating resin, such as an epoxy resin.By providing the protective film 74, the environmental characteristicsof the boundary acoustic wave device 61, such as moisture resistance,can be enhanced.

FIG. 23 is a perspective view of the boundary acoustic wave device 61shown in FIG. 22, from which the protective film 74 and the externalconnection electrode 73 are omitted. As clearly shown in FIGS. 22 and23, in the boundary acoustic wave device 61, the second medium layer 66,the third medium layer 68, and the sound absorbing layer 7 are stackedin such a manner that their external side surfaces having the wiringelectrode 72 form steps. In other words, the regions of the externalside surfaces having the wiring electrode 72 of the second medium layer66, the third medium layer 68, and the sound absorbing layer 7 aregradually shifted toward the middle in that order. The connectionelectrode 67 and the wires 69 and 71 are extended to the steps. Thus,the wiring electrode 72 is electrically connected to the connectionelectrode 67 and the wires 69 and 71 at large areas with reliability.

In the manufacturing of the boundary acoustic wave device 61 having thesteps, many boundary acoustic wave devices 61 are formed on a motherwafer, subsequently external connection electrodes 73 are formedpreferably simultaneously by photolithography, screen printing, orplating, and then the wiring electrodes 72 are provided. Finally, themother wafer is cut into individual boundary acoustic wave devices 61.Thus, the wiring between the layers can be efficiently formed at a lowcost.

FIG. 24 is a schematic front sectional view of a boundary acoustic wavedevice according to another preferred embodiment of the presentinvention. The boundary acoustic wave device 81 of this preferredembodiment has an IDT 3 and reflectors 4 and 5 on a LiNbO₃ first mediumlayer 2. The electrode structure including the IDT 3 and the reflectors4 and 5 is covered with a second medium layer 6. The second medium layer6 is preferably made of a SiO₂ film.

The second medium layer 6 is provided with a thermally conductivematerial layer 82 having a lower linear expansion coefficient and ahigher thermal conductivity than the LiNbO₃ substrate, on its uppersurface. The thermally conductive material layer 82 in the presentpreferred embodiment is made of a diamond-like carbon thin film. A soundabsorbing layer 7 is provided on the upper surface of the thermallyconductive material layer 82. The sound absorbing layer 7 is preferablymade of the same material as the sound absorbing layer 7 in the firstpreferred embodiment.

Further, an epoxy resin layer 83 is provided on the upper surface of thesound absorbing layer 7. The epoxy resin layer 83 is provided withwiring electrodes 84 and 85 on its upper surface. The wiring electrodes84 and 85 are covered with a protective film 86. The epoxy resin layer83, the wiring electrodes 84 and 85, and the protective film 86 areprovided for disposing a wiring circuit on the upper side of theboundary acoustic wave device 81.

The protective film 86 is made of the same material as the protectivefilm 74, such as an epoxy resin, and is intended to enhance the moistureresistance of the upper portion of the boundary acoustic wave device 81.

In the boundary acoustic wave device 81 of the present preferredembodiment, the thermally conductive material layer 82 on the uppersurface of the second medium layer 6 facilitates heat dissipation andprevents a temperature increase when high power is applied. Thus, theelectrical power resistance of the boundary acoustic wave device can beenhanced.

In addition, the sound absorbing layer 7 suppresses undesired spuriousresponses effectively, as in the boundary acoustic wave device of thefirst preferred embodiment of the present invention.

The thermally conductive material layer 82 can be formed of anappropriate material having a lower thermal expansion coefficient and ahigher thermal conductivity than the material of the first medium layer2, as mentioned above.

The variation of the characteristics by temperature changes of theboundary acoustic wave device of the present preferred embodimentdepends on the variations per unit temperature in acoustic velocity andin length in the propagation direction of the substrate. If theexpansion and contraction of the substrate depending on temperature isreduced, frequency variation by temperature changes can be reduced.Accordingly, a linear expansion coefficient material layer having alower linear expansion coefficient than the first medium layer of theboundary acoustic wave propagating substrate can be disposed between thefirst and the second medium layer, or on the surface of the first or thesecond medium layer. Thus, the expansion and contraction of the firstand/or the second medium layer can be reduced and the variation of thecharacteristics resulting from temperature changes can be reduced. Theabove-mentioned diamond-like carbon is an example of the materialscapable of forming the low linear expansion coefficient material layer.

Also, by reducing the thermal expansion in the direction parallel to theinterface in the boundary acoustic wave device, stresses resulting froma difference in the thermal expansion coefficient produced when thedevice is mounted on a ceramic mounting board having a low thermalexpansion coefficient can be reduced and, consequently, breakage causedby the stress can be prevented. Even if a layer having a linearexpansion coefficient with an opposite polarity to that of the first andsecond medium layers is disposed on the surface of the first and/or thesecond medium layer, the boundary acoustic wave device and the mountedstructure can be prevented from breaking by eliminating the stressresulting from the difference in thermal expansion coefficient.

Specifically, by replacing the thermally conductive material layer 82shown in FIG. 24 with the above low linear expansion coefficient or amaterial layer having a linear expansion coefficient with an oppositepolarity, the mounted structure after mounting can be prevented frombeing broken by temperature changes.

The wiring circuit of the boundary acoustic wave device 81 shown in FIG.24 includes the wiring electrodes 84 and 85. In this instance, thewiring circuit including the wiring electrodes 84 and 85 may furtherinclude, for example an inductance element, a capacitance element, aresistance element, a stripline, and a microstrip filter or mixerincluding a stub and a stripline. Thus, by providing various types ofelectrodes or circuit elements on the upper surface of the epoxy resinlayer 83, a boundary acoustic wave device 81 containing a variety ofcircuits, such as a matching circuit, can be achieved. The structurecontaining such circuits allows the omission of external circuits, suchas impedance matching circuits and modulation circuits.

The circuit including the wiring electrodes 84 and 85 is not, however,necessarily provided on the upper surface of the epoxy resin layer 83.For example, it may be disposed on the surface of the second mediumlayer 6 opposite the interface, or on at least either surface of thesound absorbing layer 7. Specifically, even if the boundary acousticwave device does not have the epoxy resin layer 83 or the protectivefilm 86, it can contain various types of circuits as in the boundaryacoustic wave device 81 shown in FIG. 24.

A boundary acoustic wave device 81 having a wire bonding or a bump bondoften has lines of several tens to several hundreds of micrometersoutside. In general, the characteristic impedance of these lines differsfrom the input and output impedances of the boundary acoustic wavedevice. Accordingly, impedance mismatching is likely to cause reflectionloss or other deterioration. The circuit including the wiring electrodes84 and 85 inside the boundary acoustic wave device allows the omissionof the long lines to reduce the reflection loss. In particular,characteristics can be improved in the frequency band of higher than 1GHz by reducing the length of the lines.

The wiring electrodes, circuit element, or external connection electrodemay be stacked together with the IDT or the reflector in the thicknessdirection. Thus, the area of the boundary acoustic wave device chip canbe reduced.

Since in the boundary acoustic wave device of the present preferredembodiment, boundary acoustic waves propagate along the interfacebetween the first and the second medium layer, the propagationcharacteristics are hardly degraded even if the device is not packagedin a case. Accordingly, the boundary acoustic wave device is notnecessarily packaged for short-term use.

For long-term use in, for example, cellular phones, however, theexternal surface of the boundary acoustic wave device is preferablycovered with, for example, the protective film 74 shown in FIG. 22. Theprotective film 74 is intended to enhance the environmental resistanceand moisture resistance. It is therefore preferable that the protectivefilm be arranged so as to cover the electrodes easily affected bycorrosion, such as the IDT and the reflectors, and regions that areeasily cracked, such as those around through holes. The protective film74 can prevent the corrosion of the SiO₂ film forming the soundabsorbing layer or the corrosion of the electrode by corrosive gases, orenhance the moisture resistance.

The protective film may be formed of, for example, a multilayercomposite of a metal and a resin, a synthetic resin layer, and a metalmaterial layer. For example, the protective film can be formed bydepositing a metal material layer, such as an Au layer, a Ni layer, andan Al alloy layer, or a Au/Ni/AlN, AlN, or Al₂O₃ layer, and thencovering the metal material layer with a synthetic resin.

Alternatively, the protective film may be formed by depositing a metalmaterial layer by a thick film forming method and then depositing asynthetic resin on the metal material layer.

FIG. 25 is a front sectional view of a boundary acoustic wave deviceaccording to another preferred embodiment of the present invention. Theboundary acoustic wave device 90 has electrodes 91 a and 91 b on thelower surface of a boundary acoustic wave device chip 91. The boundaryacoustic wave device chip 91 preferably has the same structure as theabove-described surface acoustic wave devices, but is simply indicatedby hatching in FIG. 25.

The electrodes 91 a and 91 b are bonded to electrodes 93 a and 93 b on aceramic substrate 93 with Au bumps 92 a and 92 b, respectively. Thebumps 92 a and 92 b are bonded onto the electrodes 91 a and 91 b byultrasonic bonding. After bonding the bumps 92 a and 92 b, a resinprotective film 94 covers the boundary acoustic wave device chip 91. Theprotective film 94 may be provided after the mounting of the boundaryacoustic wave device chip 91 on the ceramic substrate 93, as describedabove. In this instance, the protective film 94 is expected to reducethe stress placed on the boundary acoustic wave device chip by theceramic substrate 93.

The ceramic substrate 93 is made of a material that is harder than theboundary acoustic wave device chip 91. More specifically, it is made ofa material harder than the multilayer composite including the mediumlayers and the dielectric layer of the boundary acoustic wave devicechip 91. The electrodes 93 a and 93 b are electrically connected toterminals 93 c and 93 d provided on the bottom surface. The electrode 93a is extended to the bottom surface through the side surface of thesubstrate 93 and electrically connected to the external terminal 93 c onthe bottom surface. The other electrode 93 b is connected to the otherexternal terminal 93 d on the bottom surface with a through-holeelectrode 93 e. The connection between the external terminal on thebottom surface of the substrate 93 and the electrode on the uppersurface may be established with a through-hole electrode.

In the boundary acoustic wave device 90 of the present preferredembodiment, the boundary acoustic wave device chip 91 is joined to theceramic substrate 93 with the Au bumps 92 a and 92 b.

Thus, the boundary acoustic wave device 90 can be surface-mounted on aprinted circuit board or the like with the external terminals 93 c and93 d. In this instance, even if the printed circuit board is warped bytemperature changes, the ceramic substrate 93 stops the stress from theprinted circuit board, consequently preventing the stress from beingtransmitted to the boundary acoustic wave device chip 91. Thus, in theboundary acoustic wave device chip 91, the frequency characteristics arenot easily degraded, and the chip is not easily cracked.

The electrodes 91 a and 91 b may be made of an appropriate metal, suchas Au, Ni, or Al, or a composite composed of electrode layers of thesemetals.

FIG. 26 is a front sectional view of a boundary acoustic wave deviceaccording to another preferred embodiment of the present invention. Thisboundary acoustic wave device 96 has electrodes 91 a and 91 b on thelower surface of the boundary acoustic wave device chip 91. Thisstructure is the same as that of the boundary acoustic wave device 90.However, the boundary acoustic wave device 96 is different in thatexternal terminals 98 a and 98 b are bonded to the electrodes 91 a and91 b on the lower surface of the boundary acoustic wave device chip 91through portions 97 a and 97 b of a conductive paste, and in that areinforcing resin layer 99 is provided around the portions 97 a and 97 bof the conductive paste.

The conductive paste of the portions 97 a and 97 b preferably includes aresin adhesive and conductive powder, and is relatively soft even afterbeing cured. Accordingly, when the device is mounted on a printedcircuit board using the external terminals 98 a and 98 b, stress fromthe printed circuit board is reduced by the conductive paste portions 97a and 97 b. Specifically, the conductive paste portions 97 a and 97 bfunction as stress absorbers. Consequently, the characteristicdegradation and cracks of the boundary acoustic wave device chip 91 donot easily occur.

If the conductive paste portions 97 a and 97 b are relatively soft, areinforcing resin layer 99 shown in FIG. 26 is preferably provided. Ifthe conductive paste portions 97 a and 97 b are sufficiently hard andcan reduce the stress, the reinforcing resin layer 99 is not necessarilyrequired. The reinforcing resin layer 99 is preferably made of, forexample, an epoxy resin adhesive.

In the boundary acoustic wave device of a present preferred embodiment,since boundary acoustic waves propagate along the interface between thefirst and the second medium layer, the boundary waves have almost nomodes in which waves reach the surface of the chip. Therefore, thepackage for the boundary acoustic wave device is not required to have arecess or a hole. Specifically, packages for general boundary acousticwave devices or piezoelectric filters are required to have a recess or ahole for preventing interference with vibration. However, the boundaryacoustic wave device of the present preferred embodiments does notrequire any recess or hole even if it is packaged. Consequently, thesize can be reduced even if packaging is applied.

FIGS. 27A to 27G are front sectional views illustrating a method formanufacturing a boundary acoustic wave device.

In the present preferred embodiment, first, a mother wafer 101 isprepared. The wafer 101, which is made of about a 3 to 4 inch LiNbO₃substrate, is prepared for forming a first medium layer.

An electrode structure including an IDT 102, reflectors 103 and 104, andwiring electrodes 105 and 106 is provided on the upper surface of thewafer 101 by an appropriate method, such as a photolithography lift-offmethod.

As shown in FIG. 27B, a second medium layer 107 is arranged so as tocover the electrode structure. The second medium layer 107 in thepresent preferred embodiment is formed of a SiO₂ thin film bysputtering.

Turning then to FIG. 27C, the second medium layer 107 is etched toexpose the wiring electrodes 105 and 106 so as to be external connectingportions.

Then, a connection electrode 108 is formed so as to be electricallyconnected to the wiring electrode 106, as shown in FIG. 27D. Theconnection electrode 108 extends to the upper surface 107 a of thesecond medium layer 107.

Then, a sound absorbing layer 109 is formed by spin coating of aphotosensitive resin. The upper surface of the sound absorbing layer 109is covered with a SiN layer 110 serving as a protective film bysputtering.

Turning then to FIG. 27E, openings 111 and 112 are formed to expose thewiring and connection electrodes 105 and 108, respectively, byphotolithography etching. External terminals 113 and 114 are formedinside the openings 111 and 112 by screen printing. The externalterminals 113 and 114 are electrically connected to the wiring electrode105 and the connection electrode 108, respectively.

Thus, many boundary acoustic wave devices having the external terminals113 and 114 are formed on the mother wafer 101, as shown in FIG. 27F.The mother wafer 101 is then cut as shown in FIG. 27G, and thus manyboundary acoustic wave devices 115 are manufactured.

The manufacturing method according to the present preferred embodimentforms the sound absorbing layers 109 simultaneously while in the stageof the mother wafer 101. Consequently, variation among the soundabsorbing layers 109 of many boundary acoustic wave devices 115 can bereduced. In addition, since the sound absorbing layer 109 is made of aphotosensitive resin, the pattern of the sound absorbing layers 109 canbe easily formed with high precision. Furthermore, the SiN layer 110 asa protective film facilitates the achievement of a moisture-resistantboundary acoustic wave device.

FIGS. 28A to 28F are front sectional views illustrating a method formanufacturing a boundary acoustic wave device according to anotherpreferred embodiment of the present invention.

The method of the present preferred embodiment is the same as themanufacturing method shown in FIG. 27, except for the step of formattinga second medium layer 107. Specifically, as shown in FIG. 28A, anelectrode structure is formed on a wafer 101 in the same manner as inFIG. 27A, and then, a thin film forming the second medium layer 107 isdeposited over the wafer 101 so that sputtering particles mask theelectrode structure except the regions where external terminals areprovided. Thus, the second medium layer 107 is patterned so as to haveopenings corresponding to the openings 111 and 112 (FIG. 27E).

The following steps shown in FIGS. 28C to 28E are performed in the samemanner as the steps shown in FIGS. 27D to 27G.

The present preferred embodiment facilitates the formation of the secondmedium layer 107 having openings where the external terminals areprovided with high precision, as described above, even if the secondmedium layer 107 is formed of a material difficult to remove by etching.

FIGS. 29A to 29H are front sectional views illustrating a method formanufacturing a boundary acoustic wave device according to anotherpreferred embodiment of the present invention.

In the present preferred embodiment, a fourth medium layer 121 is formedon a mother wafer 101 (FIG. 29A). Then, the fourth medium layer 121 ispatterned. As shown in FIG. 29B, in the patterned fourth medium layer121A, the regions where an electrode structure described below is formedare defined as openings.

Turning then to FIG. 29C, the electrode structure is formed to athickness slightly smaller or equal to the depth of the openings byphotolithography. The electrode structure includes an IDT 102,reflectors 103 and 104, and wiring electrodes 105 and 106.

Turning then to FIG. 29C, a second medium layer 107 is formed. In thepresent preferred embodiment, the second medium layer is preferably madeof the same material as the fourth medium layer. However, it may beformed of another material.

Then, a third medium layer 122 is formed on the second medium layer 107,as shown in FIG. 29D. Turning then to FIG. 29E, a sound absorbing layer123 is formed on the third medium layer 122.

The third medium layer 122 is made of a Si single crystal substrate, andthe substrate is bonded to the second medium layer 107 to form thestructure shown in FIG. 29D.

Then, a sound absorbing layer 123 made of a photosensitive resin isetched by photolithography to form openings 124 and 125 shown in FIG.29F. The openings expose the wiring electrodes 105 and 106. Then,external terminals 126 and 127 are formed in the openings 124 and 125.Then, the mother wafer 101 having many boundary acoustic wave devices iscut as shown in FIG. 29G to separate the boundary acoustic wave devicesas shown in FIG. 29H.

FIGS. 30A to 30F are sectional views illustrating a method formanufacturing a boundary acoustic wave device according to anotherpreferred embodiment of the present invention.

In the present preferred embodiment, the same steps as shown in FIGS.27A to 27C are performed as shown in FIGS. 30A to 30C, thus forming IDTs102, reflectors 103 and 104, wiring electrodes 105 and 106, andpatterned second medium layer 107, for a plurality of boundary acousticwave devices on a mother wafer 101.

In the present preferred embodiment, after the patterning of the secondmedium layer 107 is finished, the mother wafer 101 is divided intoboundary acoustic wave devices by dicing, as shown in FIG. 30D. Then,external terminals 132 and 133 and a sound absorbing layer 134 areprovided on each boundary acoustic wave chip, thus producing theboundary acoustic wave device 131 shown in FIG. 30F. The sound absorbinglayer 134 in the present preferred embodiment is preferably made of anepoxy resin whose composition is adjusted so that the acoustic velocityof transverse waves is lower than that of transverse waves in the secondmedium layer, and is formed so as to cover all the boundary acousticwave devices except the exposed portions of the external terminals 132and 133. The sound absorbing layer 134 is thus formed by resin molding.

The step of forming the sound absorbing layer may be performed afterdividing the mother wafer into boundary acoustic wave devices as above.Also, the sound absorbing layer may be molded to the region except theexternal connection terminals. Thus, the environmental resistance of theboundary acoustic wave device can be enhanced.

FIG. 31 is a front sectional view of a boundary acoustic wave deviceaccording to a modification of the preferred embodiments of the presentinvention.

In the boundary acoustic wave device shown in FIG. 31, a second soundabsorbing layer 151 is provided on the lower surface of a first mediumlayer 2. This structure is the same as that of the boundary acousticwave device 1 shown in FIG. 1. The sound absorbing layer may be providednot only on the opposite surface to the interface of the second mediumlayer, but also on the opposite surface to the interface of the firstmedium layer.

Alternatively, the sound absorbing layer may be provided only on thesurface of the second medium layer, but not the surface of the firstmedium layer.

While the present invention has been described with respect to preferredembodiments thereof, it will be apparent to those skilled in the artthat the disclosed invention may be modified in numerous ways and mayassume many preferred embodiments other those specifically set out anddescribed above. Accordingly, it is intended by the appended claims tocover all modifications of the present invention which fall within thetrue spirit and scope of the present invention.

What is claimed is:
 1. A method for manufacturing at least one boundaryacoustic wave device, comprising the steps of: forming an electrode on afirst medium layer; forming a second medium layer so as to cover theelectrode on the first medium layer; and forming a sound absorbing layeron an external surface of the second medium layer; wherein the soundabsorbing layer has an acoustic velocity of transverse waves that islower than an acoustic velocity of transverse waves of the second mediumlayer.
 2. The method for manufacturing at least one boundary acousticwave device according to claim 1, wherein the step of forming the soundabsorbing layer includes the step of removing gas contained in the soundabsorbing layer.
 3. The method for manufacturing a boundary acousticwave device according to claim 1, further comprising forming acontinuously connected plurality of boundary acoustic wave devices, anddividing the continuously connected boundary acoustic wave devices intoindividual boundary acoustic wave devices before the step of forming thesound absorbing layer.